This invention relates to a method of electrically charging or discharging a member and a discharging device using the same, which are usable with electrostatic recording, electrophotography and the like.
In the field of electrophotography and electrostatic recording, corona chargers and dischargers are known and widely used, in which a high voltage is applied to a fine wire of a diameter 0.1 mm, for example, to produce corona discharge. However, they involve a drawback that the wire is easily broken because it is thin. Also, the wire is easily stained or contaminated with dust, which results in non-uniform corona production, and therefore, non-uniform charging or discharging of a member to be charged or discharged. In addition, a conductive shield which encloses the corona wire must be remote therefrom by a certain distance, so that there is a limitation in reducing the size of the device.
Another type of discharger has been proposed, as disclosed in U.S. Pat. No. 4,155,093 corresponding to Japanese Laid-Open patent application No. 53537/1979, wherein the dielectric member is sandwiched by two electrodes. By applying alternating voltage between the electrodes, positive and negative ions are produced at the junction between the dielectric member and one of the electrodes. Of these ions, the ions of a desired porality are extracted by an external electric field. This type of discharger is advantageous in that its size can be much reduced by making the dielectric member thin (not more than 500 microns, preferably 20-200 microns), and also in that a stabilized discharge can be produced with a lower voltage (peak-to-peak, approx. 1.5-2.5 KV) than the conventional corona discharge devices.
This is because the thickness of the dielectric member is small so that the strength of the electric field can be increased between the two electrodes sandwiching the dielectric member even if the alternating voltage applied therebetween is lower. The discharging can occur, if the electric field strength is high, at an edge of one of the electrodes (discharging electrode), is large enough to produce a discharging. Then, a surface discharge takes place on the surface of the dielectric member which contacts the discharging electrode.
In this type of discharger, the discharging electrode is not so stained or dusted as in the conventional corona discharges, where the discharging wire can be easily stained or contaminated with dust in the air which attaches to the wire.
However, the inventors have found that the discharges of the above described type can also be stained or contaminated with dust during use. This adversely affects the surface discharge so that the extension of the surface discharge in the direction of the width becomes not uniform along the length of the discharger, or that the degree of the extention is different from the initial or intended one. So, it has been difficult to provide a stable and uniform discharging at all times.